In the case of high voltage conversion ratios or where isolation requirements are required, bidirectional DC-DC converter circuits with function of isolation must be employed. The known circuit architectures include dual active full-bridged converter, LLC-SRC converter, two-stage series-connected converter and phase-shifted full-bridge converter using push-pull current source.
As shown in FIG. 1, the principle of the dual active full-bridged converter is to use the phase shift of the first-order side and second-order side to control its power flow direction.
As shown in FIG. 2, the LLC-SRC converter circuit architecture is composed of LLC resonant circuit and SRC (series) resonant circuit. The power flow of the bidirectional power is controlled by using a frequency conversion method. The shortcoming of this circuit is that the frequency variation range will increase with the increase of the operating voltage and is more difficult to control at low power.
As shown in FIG. 3 and FIG. 4, FIG. 3 and FIG. 4 respectively show that the later stage circuit in FIG. 1 and FIG. 2 connected in series with a buck-boost converter, so as to adapt to larger operating voltage ranges. FIG. 3 can be referred to as a two-stage dual active full-bridge series-connected buck-boost converter. FIG. 4 can be referred to as a two-stage LLC-SRC series-connected buck-boost converter. Both FIG. 3 and FIG. 4 belong to the two-stage series-connected converter which will reduce its overall efficiency and increase the cost.
As shown in FIGS. 5A and 5B, adding a push-pull current source to the first-order side of the converter and combining the phase-shifted full bridge of the second-order side may make it becomes a phase-shifted full-bridge converter using a push-pull current source, wherein FIG. 5A shows an aspect of using a snubber as a switch clamp and FIG. 5B shows another aspect of using the active clamp. The circuit of FIG. 5A and FIG. 5B is only suitable for the application having lower voltage on the first-order side, but for the application having high voltage on the first-order side, the circuit design is subject to great restrictions since the voltage across the power switch needs to bear high voltage and therefore the practical application is more difficult.
In other words, since the power conversion of the bidirectional DC-DC power converter requires the variation of high voltage ratio at the input or output ends, a wide range of frequency variations must be provided if a resonant converter is used and therefore the design of the resonant circuit is more difficult and inefficient. Although the addition of buck-boost converter circuit can obtain a wide input voltage range, the known circuit is impossible to use the frequency conversion method to get a quick response and to achieve the seamless of the bidirectional power regulated mode switch. The present invention can solve the aforementioned problems by providing a resonant circuit controlled by a fixed frequency method and combined a buck-boosting conversion circuit.